The present invention relates to microelectronic connection components and more specifically relates to methods of attaching solder balls to conductive pads or terminals on a microelectronic element, such as a microelectronic connection component.
Soldered connections are typically used throughout the electronics industry to connect electronic components. Where the components to be connected include dielectric elements, such as a printed circuit board or a flexible dielectric sheet having conductive metal traces, the traces may be provided with enlarged regions, commonly referred to as “lands” or “conductive pads.” A mass of solder may be deposited on each conductive pad by exposing the assembly to a liquid solder or, more typically, by applying solder preforms commonly referred to as “solder balls” onto the pads and heating the assembly to melt or reflow the solder balls. Solder balls are typically reflowed by raising the temperature of the solder balls above a predetermined temperature, generally referred to as the melting point of the solder balls. The melting point is defined as the temperature at which the solder balls transform from a first solid or frozen condition to a second molten or at least partially liquid condition. Once the solder balls transform to the second at least partially liquid condition, the solder balls remain in the least partially liquid condition as long as the temperature of the solder material is maintained at or above its melting point. The reflowed solder typically wets to the metal of the conductive pads to form a strong bond between the pads and the solder. After the solder balls have wet to the conductive pads, the temperature of the solder balls may be reduced to a level below the melting point, whereupon the solder balls transform from the second at least partially liquid condition to the first solid condition.
After application of the solder masses, the component typically has solder masses protruding from the surface. A semiconductor chip package having an array of solder masses on a surface in a grid-like pattern is sometimes referred to as a “ball-grid array” element. The use of ball-grid arrays in packages for microelectronic devices such as semiconductor chips is described for example, in the article “TBGA Package Technology,” IEEE Transactions on Components, Packaging and Manufacturing Technology, Part B, Vol. 17, No. 4, VP 564-568 by Andros and Hammer and in “Ball Grid Array Technology,” Lau, J. H. ed, pp. 460-464.
Commonly assigned U.S. Pat. Nos. 5,148,265 and 5,148,266, the disclosures of which are hereby incorporated by reference herein, describe, in certain preferred embodiments, microelectronic packages having a set of conductive pads in the form of terminals which may be mounted on a dielectric layer such as a flexible sheet. The conductive pads may be connected to contacts on a semiconductor chip by flexible leads and may be supported above the surface of the chip by a compliant layer such as an elastomer located between the terminals and the chip, typically between the dielectric layer and the chip. Solder masses may be attached to the pads or terminals for connecting the assembly to a circuit board or other substrate having corresponding pads.
The microelectronic packages described above can be engaged with another component having a corresponding set of contact pads. After engaging the protruding solder masses with the pads of the other component, the solder masses may be heated again to melt all or part of each solder mask and bond the solder masses to the contact pads of the other component. The resulting solder columns electrically interconnect pads on both components with one another electronically and also form a mechanical connection between the components.
The presence of oxides on solder balls adversely affects the ability of the solder balls to wet to and ultimately bond with contact pads. As is well known to those skilled in the art, if an oxide layer is not adequately removed from a contact pad, then molten solder will not wet to the pad, thereby resulting in the formation of a poor bond between the molten solder and the contact pad. In order to avoid this problem, some bonding processes include the steps of removing oxides from both the solder balls and the contact pads, such as by scrubbing the surfaces of the solder balls and contact pads. Another solution for removing oxides is to use flux during the bonding process. In addition to facilitating placement of solder balls on conductive pads, flux aids in the removal of oxides that develop when the pads and solder balls are exposed to the environment.
FIG. 1A shows a flexible dielectric sheet 10 having a first surface 12 and a second surface 14 remote therefrom. The dielectric sheet is preferably a flexible polymeric material. The connection component 10 includes one or more conductive pads 18 formed over the first surface 12 of the dielectric sheet 10. The dielectric sheet 10 includes metal lined vias 20 extending between the first surface 12 and the second surface 14 of the dielectric sheet 10. The metal lined vias 20 have first ends 22 attached to conductive pads 18 and second ends 24 attached to conductive traces 26. Referring to FIG. 1A-1, the conductive traces 26A and 26B extend along the second surface 14 of the dielectric sheet 10. In the embodiment shown in FIG. 1A-1, the first conductive trace 26A extends in a first direction that is substantially perpendicular to the direction of the second conductive trace 26B.
Referring to FIG. 1B, it may be desired to bond and/or attach one or more solder balls 28 to the conductive pads 18. Before the solder balls 28 are placed atop the conductive pads 18, a flux material 30 is placed atop each conductive pad 18. The flux material facilitates placement of the solder balls 28 atop the conductive pads by temporarily holding the solder balls in place until the solder balls can be reflowed and permanently connected to the conductive pads 18. The flux material 30 also removes oxides that may have formed atop the exterior surface of the conductive pads 18. The use of flux for removing oxide layers is of critical importance as the reflowed solder 28 will be unable to wet to the surface of the conductive pads unless the oxide layer is removed therefrom. FIG. 1B-1 shows a top view of the connection component 10 of FIG. 1B. The pads of flux material 30 are provided atop conductive pads 18. Solder balls 28 are centered over the conductive pads 18 and atop the flux 30. As shown in FIGS. 1B and 1B-1, the flux 18 and solder balls 28 cover the via openings 20. As will be described in more detail below, because the via openings 20 are covered by flux 18 and the solder balls 28, air and vapors from the flux may be trapped in the vias 20 when the solder balls are reflowed. As a result, the heated air and flux vapors have no room to expand. FIG. 1C shows the connection component 10 during a solder reflow step. The solder balls 28 may comprise a tin/lead solder composition having a melting point of 320° C. When heat is applied to the solder balls 28, the flux 30 at least partially fills up the vias and the expanding hot air and/or vapors from the flux generally has no opportunity to escape through the via opening. As a result, as the air and/or vapors from the flux within the via 20 expand, the air and/or the vapors flow into the center of the liquid solder 28 to form one or more voids 32 in a central portion of the solder balls 28.
FIG. 1D shows the solder balls 28 after they have been reflowed and permanently attached to conductive pads 18. When viewing the attached solder balls from an exterior surface, the balls 28 appear to be solid. However, the centers of the balls 28 typically have one or more voids. As mentioned above, the voids may dramatically diminish the reliability of the connection component. Although the present invention is not limited to any particular theory of operation, it is believed that solder balls having voids will fail sooner than solder balls without voids. This is because the solder balls with voids have a tendency to crack during thermal cycling which may adversely affect the ability of the connection component to maintain an electrical interconnection with another circuit element. Moreover, the existence of voids in solder balls may cause the solder balls to be improperly dimensioned. Such improperly dimensioned solder balls will cause non-planarity of a package, making it difficult to place the package in a test socket or attach the package to a substrate. As a result, packages having improperly dimensioned solder balls with voids are rejected at a much higher rate than packages having properly dimensioned solder balls.
Thus, there is a need for improved methods for connecting solder balls to microelectronic elements that minimize the voiding problems described above.